The present application is based on and claims priority from Japanese Patent Application 2000-172205 filed Jun. 8, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a plane commutator that has a disk-like commutator surface and a method of manufacturing such a commutator.
2. Description of the Related Art
A plane commutator that has a disk-like flat commutator surface is installed in a motor such as a motor of a motor-integrated fuel-pump unit. The plane commutator is fitted to a rotary shaft of the motor. The plane commutator is comprised of a plurality of commutator segments (generally made of a burnt or sintered carbon) insulated by slits from each other, a plurality of metal base plates, and a resinous boss member. The plurality of the commutator surface forms the flat commutator surface. Each of the commutator segments is connected to one of the metal base plates.
There are two types of the plane commutator. The first type has the metal base plate connected to an end of the commutator and the commutator surface formed at the other end of the commutator. The second type has the metal base plate connected to portions around the same end of the commutator as the commutator surface.
In the first type, it is difficult to mechanically fix the base plates to the commutator due to spring-back force of the base plate. Therefore it is necessary to weld or solder the base plates to the commutator segments while pressing the base plate in the axial direction thereof.
U.S. Pat. No. 5,925,961 or its corresponding Japanese Application, JP-A-10-4653, discloses a method of fixing the base plate to commutator segments of the first type commutator. Each commutator segment has a projection extending in the axial direction from the surface of the commutator opposite the commutator surface. The metal base plate has holes to which the projections are inserted. The holes are thereafter crimped or pressed at the circumference thereof to mechanically fix the base plate to the commutator segments.
However, it is difficult to control the pressing force to provide a desired strength in the disclosed method. If the pressing force is not controlled as desired, the projection may be broken. This is an obstacle to mass-production of the above type of the motors.
Therefore, a main object of the invention is to provide an improved plane commutator that is easy to manufacture and more reliable.
In a plane commutator according to a feature of the invention is as follows.
A plurality of commutator segments is made of sintered carbon compound and is disposed to provide a plane commutator surface at an end thereof and a base portion at the other end. Each the commutator segments has a projection extending from the base portion. A resinous boss member is disposed in contact with the base portion. A plurality of metal base plates has an engagement hole, to which the projection of one of the commutator segments is fitted and a terminal portion extending along an outer periphery of the boss member. A powder-accommodation pocket is formed around a foot of each the projection.
When the plurality of commutator segments and the plurality of metal base plates are assembled into a unit, each the projection is inserted into an engagement hole and shaved by the engagement hole. The shaved powder shaved off the projection is accommodated in the powder accommodation pocket.
Therefore, the projection and the engagement hole can contact each other very closely, thereby increasing fastening strength as well as reducing electric contact resistance between the commutator segment and the metal base plate.
Each metal base plate may be a press-formed plate that has a dent for accommodating shaved powder formed when the projection is fitted to the engagement hole. Preferably, each engagement hole has a corner edge for shaving an outer periphery periphery of one of the projections, and the edge corner may have a curved surface whose radius is less than 0.2 mm. It is also preferable that the projection has a tapering portion at an edge thereof to insert the projection into the engagement hole more easily.
In the above plane commutator, each commutator segment may be comprised of a commutator surface portion disposed on a side of the commutator surface and a commutator base portion disposed on the side of the projection, and the commutator base portion has a lower electric resistance than the commutator surface portion. The commutator base portion may include metal powder.
Another object of the invention is to provide a new method of manufacturing the above plane commutator.
A method of manufacturing a plane commutator according to another feature of the invention includes a step of extending a projection from each commutator segment, a step of forming an engagement hole at each metal base plate so that the outside diameter of the projection is a prescribed size larger than an inside diameter of the holes, and a step of inserting each projection into the engagement hole to shave an outer periphery of the projection so that the projection can be shaved by an inner periphery of the holes.